The prior art will be described in terms of resin coated sand used in the Shell Process employed by the metal casting and foundry industry. The shell process was developed in Germany during the Second World War, and the process was used to produce molds for mortars, artillery shells and other projectiles. The Germans attempted to keep the process secret after the war; however, the process was discovered by allied investigators who placed the process in the public domain as war booty which then provided the foundry industry with a revolutionary process.
The Shell Process (also known as the Croning or C Process) is used to produce hollow light weight molds and cores for pipe hubs, cores, crank shafts, intake manifolds for engines, etc. In fact, more foundries utilize the shell process, to produce resin sand cores and molds, than any other process. The process is extensively applied worldwide.
The original Croning process blended raw sand with powdered phenolic resin and powdered hexamethylenetetramine (a curing agent or hardener) “hexa” which was gravity fed into a preheated pattern. The heat melted the resin and hardener to fuse the sand within the pattern (or mold). After a suitable thickness of sand was obtained, the inactivated sand was dumped from the pattern, leaving the hollow core sand mold. As time went by, the process was improved by pre-coating the sand with the required ingredients (resin-hardener-wax-fillers-etc.) at a sand facility. The “foundry sand” is then sold as a free-flowing product to foundries (or foundries produce their own free-flowing product).
The current state of the art uses batch mixers to coat substrates (minerals, ceramics, etc. sometimes referred to generally as industrial aggregates) with a resin(s) and other ingredients. That is, sand (aggregate) is preweighed, heated to the desired temperature and transferred into a batch mixer. Resin(s) and additives are then added sequentially and held in the mixer until the material has reached the required cure stage or begins to break down into smaller agglomerated clumps of sand (aggregate) and resin. The mixture is then dumped and the cycle is repeated. Newer mixers now use a continuous process; however, the manufacturing steps and compounds used are essentially similar.
More specifically in the current state of the art for producing coated foundry sand preweighed sand is heated to between 280° F. to 380° F. The sand is then fed into a Muller type mill (or continuous mixer) and the resin dumped in to sand. The heat from the sand melts the resin and the resin flows around the sand grains to encapsulate the grain. After sufficient mull time, liquid hexa is added to the sand and resin, generally below 280° F. The hexa/resin mix reacts slightly to begin to crosslink the coating before a water quench is added to bring the sand temperature down to a temperature typically below 200° F. This quench stops the reaction of the hexa/resin and the resin coated sand is said to be at the “B” stage. The mixture continues to mull and dry completely and break apart into resin coated sand which essentially is an encapsulation of individual sand grains. The resin coated sand is advanced to the “C” stage when the coated sand is placed into a heated tool (the mold at a foundry) at 400-700° F. This heat liberates formaldehyde and ammonia from the original hexa solution (hexa in a liquid form is a combination of ammonia (40%) and formaldehyde (60%). The liberated formaldehyde reacts further with the resin to crosslink the resin and creates a solid form or a core or mold, and the free ammonia is given off as a volatile organic gas that has an odor that is offensive to the operators and the neighboring communities.
Several instances of a curing agent chosen to reduce emissions of ammonia appear in the prior art. Gardziella et al. disclosed a “Novel Heat-hardenable Binders Phenol-formaldehyde+HMT+Acid” in U.S. Pat. No. 4,942,217. Gardziella still used hexa as their curing agent but stated that the resin compounds and binders helped reduce emissions. An example of the composition used for “hot bake” (shell or Croning) casting sands was given.
Geoffrey et al., in U.S. Pat. No. 5,189,079, disclose a “Low Free Formaldehyde Phenolic Polyol Formulation” in which the inventors recognize the need to reduce the odor of formaldehyde in urethane binders which are used in the ‘cold-box’ and ‘no-bake’ core casting sand processes.
Johnson et al. disclosed a “Benzoxazine Polymer Composition” in U.S. Pat. No. 5,910,521 recognizing the need to cure novalac resins without the emission of ammonia. Johnson et al. disclose the use of their compound in foundry sand; however, their examples teach mixing of powdered resin with their powdered curing agent with the foundry sand. Johnson et al. state that their curing polymer may be a solid at room temperature and will take the form of a powder. However, they add that if the water removal is controlled during the manufacturing process, then the curing polymer may be produced in liquid form.
Waitkus et al. disclosed a “Polymer Composition for Curing Novalac Resins” in U.S. Pat. No. 6,569,918 also recognizing the need to reduce ammonia emissions. Waitkus et al., like Johnson et al., also disclose the use of their compound in foundry sand and with silica sand (proppants); however, unlike Johnson et al., the Waitkus examples disclose the addition of their curing agent as a liquid—a suspension in methanol—well after the sand is coated with the novalac resin (as a strict laboratory experiment). It should be remembered that the Waitkus compound includes odor producing ingredients.
Thus, there remains the need for resin-coated casting sand, or in general resin coated industrial aggregates, that reduces or eliminates offensive odors while keeping the required free flowing characteristic until the resin is activated in the mold.